What Causes 'ZgotmplZ' in Go HTML Templates and How Can It Be Resolved?

Unveiling the Mystery of "ZgotmplZ" in Go HTML Templates

When working with Go HTML templates, you may encounter the enigmatic "ZgotmplZ" in your output. This is not a simple string but a special value that signifies an issue.

Understanding ZgotmplZ

"ZgotmplZ" stands for "Zero-value got templateZ," and it appears when unsafe content is passed to a CSS or URL context in your template. For instance, executing the following template:

<option {{ printSelected "test" }} {{ printSelected "test" | safe }}>test</option>

will output:

<option ZgotmplZ ZgotmplZ >test</option>

Resolving the ZgotmplZ Issue

The key to addressing "ZgotmplZ" is to ensure that only safe strings are used in insecure contexts. This can be achieved by applying the safe function to the output of template functions. However, in the case of attributes like selected, directly applying safe may introduce XSS vulnerabilities.

To address this, you can add a custom attr function to your template funcMap:

funcMap := template.FuncMap{
    "attr": func(s string) template.HTMLAttr {
        return template.HTMLAttr(s)
    },
    "safe": func(s string) template.HTML {
        return template.HTML(s)
    },
}

With this function in place, you can modify the template as follows:

<option {{.attr | attr}}>test</option>
{{.html | safe}}

This ensures that the attribute value is safely escaped, while allowing HTML content to be rendered as raw.

Conclusion

By understanding the purpose of "ZgotmplZ" and employing appropriate escaping techniques, you can prevent unsafe content from reaching insecure contexts in your Go HTML templates, maintaining the integrity and security of your web applications.

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